The term “battery” originally meant a plurality of electrochemical cells connected in series. However, nowadays, the term “battery” is very frequently, including in the following description, also used to refer to individual electrochemical cells (single cells). Batteries that are not rechargeable are referred to as primary batteries or primary cells.
Lithium batteries are frequently cells that use lithium as active material in the negative electrode. Examples of batteries having a negative lithium electrode are the lithium-thionyl chloride battery, the lithium-sulphur dioxide battery, the lithium-carbon monofluoride battery, the lithium-iron sulphide battery and the lithium-manganese dioxide battery. As a positive electrode, use is made in the last-mentioned lithium-manganese dioxide battery of manganese dioxide electrodes, iron disulphide electrodes in a lithium-iron sulphide battery and, in a lithium-sulphur dioxide battery, sulphur dioxide dissolved in acetonitrile (absorbed in an inert electrode). The positive and negative electrodes of a lithium battery are separated from one another by a separator. This generally allows ion transport between the electrodes. Ion transport is facilitated by use of a suitable electrolyte.
An electrolyte suitable for a lithium battery typically consists of a solvent and a lithium salt having a large anion, for example, lithium hexafluorophosphate.
The solvent consists of two or more solvent components.
As a first solvent component, the solvent generally comprises an organic carbonate, in particular from the group consisting of propylene carbonate (PC), ethylene carbonate (EC) and butylene carbonate (BC), or an ester, in particular from the group consisting of γ-butyrolactone (GBL), ethyl acetate (EA) and methyl formate (MF). This component displays a very high ionic conductivity and a high stability to metallic lithium. However, it is relatively viscous.
As a second solvent component, the solvent comprises, for example, dimethoxyethane (DME), ethyl methyl carbonate (EMC) dimethyl carbonate (DMC) or diethyl carbonate (DEC). Such compounds have a comparatively low viscosity relative to the first component. Their presence thus decreases the overall viscosity of the electrolyte. On the other hand, its ionic conductivity is increased.
DME is particularly suitable as second component for primary lithium batteries. A particularly preferred solvent for primary lithium batteries typically consists of PC and DME. It is used both in button cells and in round cells.
There have for some time been indications that DME is toxic to reproduction. For this reason, the European Chemical Agency (ECHA) has decided to put DME on the “list of substances of very high concern.” Accordingly, future approval of DME is uncertain.
It could therefore be helpful to provide primary lithium batteries having electrolyte free of DME. The replacement of DME should as far as possible not be associated with disadvantages concerning the functionality of the electrolyte.